The following adjustments are made in order to modernise
bring the role into line with the other mainline roles:
1. Simplify the constraints mechanism.
2. Add the developer mode constraints functionality.
3. Clean up developer mode logic to allow pip installs
into an existing venv if the deployer chooses to do so.
4. Normalise the distro package install task to make it
use the same name, to add retries and ensure that the
cache updates appropriately.
5. Clean up some commented vars and tasks which were not
used.
6. Simplify the use of checksums for the venv downloads
to use modern Ansible functionality.
7. Add additional python venv prep for SuSE/CentOS.
8. Add the recording of the current venv tag deployed.
Change-Id: I9daa4352aa818db03f682eb0d1a65eefff9bb6f6
d4acca1ce9